1. Field of the Invention
The present invention relates to tools for facilitating orthopedic procedures. More particularly, this invention pertains to a tool for reliably centering a drill bit within the femoral canal for facilitating removal of the plug of cementous material that remains after removal of a worn hip prosthesis.
2. Description of the Prior Art
While primary total hip arthroplasty (THA) generally produces a successful and long-lasting result, annually about one percent of existing hip replacements require revision. FIG. 1 is a side elevation view in partial cross-section of the region of a hip replacement showing a prosthesis 10 fixed within a human femur 12. The prosthesis may be fabricated of, for example, stainless steel and includes a metal ball or prosthetic head 14 that is, in turn, coupled to a prosthetic hip socket (not shown). The lower portion of the prosthesis 10 extends to and terminates in a elongated stem 16. The stem 16 extends into the femoral canal.
The stem 16 is anchored to the surrounding bone of the femoral canal by means of cement 18. The hardened cement 18 (a mixture of a monomer and a polymer) surrounds the stem 16 and extends beneath, forming a solid region known as a cement “plug”. A stop or restrictor 22 of plastic may have been inserted within the femoral canal during the procedure to increase backpressure (and thereby improving the quality of the cement-to-bone tissue bond) on the cement during hardening.
A gradual breakdown or loosening of the bond between the cement that holds the prosthesis within the patient's femur and the surrounding bone generally occurs over time, limiting the useful life of the existing procedure to about 10 to 20 years. Such loosening of the bond leads to the patient's experiencing pain. When such pain becomes significant, revision is required. This involves removal of the existing prosthesis and attachment of a new prosthesis in its place. The required surgical procedure involves removal of existing cement and the optional restrictor, in addition to the existing prosthesis, prior to insertion of a new device. The subsequent hip replacement often employs a non-cement fixation technique due to the thinning and reduced adhesion of existing bone tissue. The latter prosthesis may, for example, be formed with beaded or mesh-like surfaces that encourage interdigitation of bone tissue thereon. An alternative technique is to inject pressurized cement into a femoral canal that has been cleared of all debris. Generally, revision hip replacement can achieve a long lasting result and provide substantial pain relief.
The revision surgery is begun by removal of all foreign objects from the femoral canal. The existing prosthesis is readily removed, leaving the mantle of hardened cement and, possibly, the plastic restrictor in the canal. Removal of the structures associated with the existing prosthesis becomes more complicated and risky as one progresses further and further from the top or head of the bone into the femoral canal. While the uppermost cement is not as difficult to remove, visibility and accessibility limitations crop up as the surgeon proceeds further. Maintenance of adequate lighting is difficult and further obscuration is caused by the increased presence of blood once the upper regions of the cement have been removed (by, for example, power burrs, hand-held chisels, ultrasound, etc.), leaving the cement plug (and, perhaps, the plastic restrictor).
Removal of the remaining plug (and optional restrictor) is commonly accomplished by a number of tools and techniques including elongated drill bits, burrs or hand-held chisels to break through, fragment or create a hole in the plug. An elongated tool having a hooked end can then be inserted through the obstruction and an upwardly-directed force applied to lift and remove it.
The process of making a hole through the cement plug with an elongated drill bit is complicated by an environment that is obscure, small, remote and adjacent bone tissue whose integrity must be protected. Visibility and alignment difficulties in the plug region create a very dangerous environment in which to operate a drill bit, burr or chisel. Misalignment may result in the perforation of bone tissue. X-ray or like equipment is often employed to assure proper centering of potentially-harmful tools within the femoral canal. Another method of obtaining visibility is the excising of a “window” in the bone. This complicates the surgery, increasing its duration, the loss of blood and the risk of subsequent bone fracture.
U.S. Pat. No. 5,649,930 of Kertzner et al. covering “Orthopedic Centering Tool” teaches a tool for guiding a surgical drill bit through a cement plug within the femoral canal. Such tool provides a means for centering a drill bit that is not dependent upon direct observation of the drill bit within the interior of the femoral canal during surgery. Rather, it enables the surgeon to be assured that the bit is properly directed through an alignment procedure that occurs before the drill bit enters the canal.
The patented tool includes an adjustable frame comprising a pair of right angle sections mounted in a mirror image relationship. The sections are adjustably clamped to one another and, in turn, secure a vertical sleeve for guiding a surgical drill bit and a horizontal sleeve for accommodating an anchor pin. Various clamps permit a surgeon to adjust the tool so that the drill bit is guided to the approximate center of the femoral canal immediately below the plug while the pin anchors the frame to the bone.
While providing a number of advantages over such procedures as x-rays and the cutting of a window into the bone, the above-described tool is somewhat awkward to adjust during an operation. Such adjustment is undertaken within a difficult environment that includes blood, cement and bone fragments. Time limitations also complicate use of the tool. They may be due, for example, to the patient's tolerance for anesthesia. These and related factors complicate the surgeon's ability to perform the multiple adjustments required to assure that the drill bit has been positioned properly with respect to the bone.